A microporous sheet or film is useful as a printing substrate, such as synthetic paper; as a substitute for leather; as a highly filbrillated sheet which can easily be shredded into fine fibrils to be used as substitutes for paper-making pulps, or as a filter material, such as battery spearators.
Porous paper having sufficient wet-strength and impregnated with nicotinic acid, such as disclosed in U.S. Pat. No. 2,491,646 is used as a meat-interleaf paper. In industry, meat-interleaf paper is stacked between freshly cut steaks to prevent formation of dark brown color, and preserve the fresh red color meat during storage. The function of the porosity of the paper is to make small amounts of oxygen available to the meat pigment, myoglobin; oxygen is needed to retain the myoglobin at the red state. The chemical additive, nicotinic acid, prolongs the time for which the red color is retained by a catalytic mechanism not entirely understood. Other chemicals are known from the patent literature to produce similar effects (U.S. Pat. Nos. 3,867,558: gamma-pyrone; 3,615,691; tetrazole; 3,266,909: glutamic acid salt; 2,863,777: pyridine/ascorbic acid).
Many polymeric materials or especially blends thereof are known to undergo fibrillation and/or pore formation upon stretching or drawing. A number of such blends are described in U.S. Pat. Nos. 3,697,367 to Schwarz and 3,511,742 to Rasmussen. Such pore formation may result from different causes, such as separation of phases of incompatible polymer blends, or separation or inorganic polymer fillers like clay or titanium dioxide from the polymer matrix due to stress concentration. Most common in such systems is that the maximum pore formation effect occurs at a draw temperature which is relatively low for the particular polymer system. When the same polymer or blend thereof is stretched at higher temperatures, the pore formation diminishes and a denser film results.
At temperatures where pore formation occurs accompanied by a decrease in density, the draw tension also increases. Draw tension or yield strain also increases with increasing draw rate or operating speed, and reaches the breaking strength of the base film at speeds which are slow and uneconomical for conventional systems used for stretching or drawing of films. Operating a conventional stretching system, such as longitudinal stretching by Godet rolls and lateral stretching by tenter frames, under tensions which approach the breaking strength of the base film often causes breaks and frequent interruptions of the process. Extrusion speeds are uneconomically slow: for instance, an acceptable draw rate of 200 cm/min in a single longitudinal draw step over Godet rolls for a 90 wt% isotatic polypropylene--10 wt% polystyrene (See Example 1), would limit the extrusion rate (for a 3' linear die at a draw ratio of 2.0 and a film thickness of 100 micron) to 23.2 lb/hr.
In copending application Ser. No. 614,018, there is disclosed a method for fibrillating polymer blends of incompatible polymers or filled polymers to form fibrillated or microporous structures by cold drawing at high tension such blends or polymers. Such microporous structures are very difficult to impregnate with solvents or solutions of chemicals because of the small size of the pores. Surface tension and capillary action prevent the entry of solutions. Thus, a microporous film made by the method of such parent application while usually permeable to vapors is not permeable to liquids. Permeability indicated complete open-cell pore structure of the fibrillated films.